Conversion of ac power (such as from ac lines from utilities or generators and alternators) to dc power has typically been done by using a diode bridge rectifier and a filter capacitor connected across the rectifier output in parallel with a load. A typical single-phase ac-to-dc diode bridge rectifier circuit is shown in FIG. 1. In FIG. 1, the rectifier 10 is fed by a single-phase ac line input 12 which is connected across the ac input terminals of a diode bridge 14. The rectified dc output from diode bridge 14 is fed to a load, modeled in FIG. 1 as a resistor R.sub.L. A filter capacitor C.sub.o is connected across the load to smooth the dc current to the load. Parasitic line inductance associated with this type of circuit is shown as a lumped inductor L.sub.i.
The circuit shown in FIG. 1 has the disadvantage of generating pulsed ac line currents drawn from the ac lines. The non-ideal character of the input currents from the ac lines creates several problems for the power distribution network and for other apparatus in the vicinity of the rectifier. Among the problems are high input-current harmonic components, low rectifier efficiency because of the large rms value of the input current, input ac line voltage distortion because of the associated peak currents, and a maximum input power factor much, much less than unity (e.g., 0.5 to 0.75), due primarily to the presence of a third harmonic component of the ac frequency of considerable amplitude.
A modified rectifier circuit to increase power factor and reduce harmonics has been proposed as shown in FIG. 2. In FIG. 2, rectifier circuit 16 is fed by a single-phase ac line input 18 which is connected across the ac input terminals of a diode bridge 20. The rectified dc output from diode bridge 20 is fed to a load, modeled in FIG. 2 as a resistor R.sub.L. A filter capacitor C.sub.o is connected across the load to smooth the dc current to the load. An input filter circuit 22 is located in series with one leg of the ac line input 18. Input filter circuit 22 consists of an inductor L.sub.r and a capacitor C.sub.r connected in parallel.
The circuit shown in FIG. 2 is stated to have advantages over the circuit of FIG. 1. Input peak current is said to be lower, reducing input voltage distortion. Power factor is said to be increased somewhat, to 0.887. Efficiency is said to be increased because of the low rms values of the input current. However, the circuit of FIG. 2 is admittedly more complex in operation than the circuit of FIG. 1.
Other proposed solutions to improve power factor and reduce line current harmonics have taken the approach of using passive circuits to correct line current harmonics and separate passive circuits to increase power factor, but this approach has not yielded acceptable performance levels. Still other proposed solutions have taken the form of active circuits. These active circuits do work well, but require a large number of components, and are not very rugged in terms of line transient susceptibility and damage from overload or short circuit.
The present invention overcomes the drawbacks of the prior solutions, and uses a passive circuit which reduces line current harmonics and increases power factor using only three passive components. The circuit can be tuned to reduce total line current harmonics or total harmonic distortion (THD) to below 5%, while increasing power factor to greater than 0.95. The present invention also provides start up power for other circuitry, such as a switch-mode power supply which may form part of the load R.sub.L, without the need for additional active or passive components. The present invention significantly increases power factor, reduces THD, is rugged and reliable, and is simple and inexpensive to construct.
Other objects and advantages of the invention will appear hereinafter.